Scheman

Gregor Mendel

• Austrian Monk, mid- 19th century.

• Worked with Pea plants.• 1st to succeed in predicting the

passage of traits from parent to offspring.

• Utilized Scientific Method with controlled experimentation.

Both Stamens and Carpels

Control cross-pollination

• What genetic principles account for the passing of traits from parents to offspring?

• The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)

• The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)

• Mendel documented a particulate mechanism through his experiments with garden peas

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

Particulate Hypothesis Proven (no blending)

Mendel’s Key Points• Hybrid = The offspring of

parents that have different forms for the same trait.

• Two Factors that control gene activation for any particular trait. (Alleles)

• Dominant vs. Recessive• Law of Segregation• Law of Independent

Assortment

•What Mendel called a “heritable factor” is what we now call a gene!

• Mitosis and meiosis were first described in the late 1800s

• The chromosome theory of inheritance states:– Mendelian genes have specific loci (positions) on

chromosomes

– Chromosomes undergo segregation and independent assortment

• The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

0.5 mm

Meiosis

Metaphase I

Anaphase I

Metaphase II

Gametes

LAW OF SEGREGATIONThe two alleles for each gene separate during gamete formation.

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.

14yr 1 4

Yr14 YR

3 3

F1 Generation

14yR

R

R

R

R

RR

R

R R R R

R

Y

Y

Y Y

Y

YY

Y

YY

YY

yr r

rr

r r

rr

r r r r

y

y

y

y

y

y y

yyyy

All F1 plants produceyellow-round seeds (YyRr)

1

2 2

1

Punnett Squares

• Technique for predicting offspring from one generation to the next.

• Homozygous vs. Heterozygous

The Testcross• How can we tell the genotype of an individual

with the dominant phenotype?• Such an individual must have one dominant

allele, but the individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

Phenotypic ratio approximately 9:3:3:1315 108 101 32

Concept 14.3: Inheritance patterns are often more complex than predicted by simple

Mendelian genetics

• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

• Many heritable characters are not determined by only one gene with two alleles

• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:– When alleles are not completely dominant or

recessive– When a gene has more than two alleles– When a gene produces multiple phenotypes

Degrees of Dominance

• Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

incomplete dominance

Multiple Alleles

• Most genes exist in populations in more than two allelic forms

• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

• The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Blood Typing

IA

IB

i

A

B

none(a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

• Most genes have multiple phenotypic effects, a property called pleiotropy

• For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

• In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

• For example, in mice and many other mammals, coat color depends on two genes

• One gene determines the pigment color (with alleles B for black and b for brown)

• The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

• Quantitative characters are those that vary in the population along a continuum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees

Punnett Square Practice

Great Punnett Square Problem Web Site

www.biology.arizona.edu

Go to Mendelian Genetics

Meiosis• The process of making

gametes (sex cells such as sperm and egg).

• Purpose = to reduce the chromosome number in half!

• 1 Diploid(2N) cell becomes 4 Haploid(N) cells.

Human Meiosis• Spermatogenesis

produces 4 viable sperm every meiosis.

• Oogenesis produces 1 viable egg every meiosis and 4 polar bodies.

• Zygote = fertilized egg which contains the standard number of chromosomes.

46

Zygote

• Most of the time, meiosis occurs flawless.

• However, sometimes, chromosomes, parts of chromosomes, or specific nucleotides get messed up!

• Genetic disorders are inherited from the parents and can be found in the DNA of every cell.

Nondisjunction

• Non disjunction results when an entire chromosome does not separate from its sister chromosome.

• Thus, both chromosomes travel together.

Trisomy 21

Trisomy 21

Cystic Fibrosis

• Common disorder in Caucasians.

• Mutation in amino acids that make up a specific transport protein.

• Thick mucus builds up in respiratory and digestive systems.

Sickle Cell Anemia & PKU

• Minor changes in the sequence of nucleotides can lead to extreme genetic disorders.

• Sickle cell anemia• PKU

Sex-Linked Inheritance

• Some inherited traits are located on the X and Y (sex) chromosomes.

• Hemophilia• Pattern baldness• Color blindness• Duchenne muscular

dystrophy

Dominant Inheritance

• 6-Fingers• Huntington’s Disease

Morgan’s Experimental Evidence: Scientific Inquiry

• The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist

• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair

• In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)– The F1 generation all had red eyes– The F2 generation showed the 3:1 red:white eye

ratio, but only males had white eyes• Morgan determined that the white-eyed mutant allele

must be located on the X chromosome• Morgan’s finding supported the chromosome theory of

inheritance

EggsF1

CONCLUSION

Generation

Generation

P XX

w

Sperm

XY

+

+

++ +

Eggs

Sperm+

++ +

+

GenerationF2

w

w

w

ww

w

w

w

ww

w

w

w

ww

44 + XY

Parents 44 + XX

22 + X

22 +X

22 + Yor +

44 + XX or

Sperm Egg

44 + XY

Zygotes (offspring)

(a) The X-Y system

22 + XX

22 + X

(b) The X-0 system

76 + ZW

76 + ZZ

(c) The Z-W system

32(Diploid)

16(Haploid)

(d) The haplo-diploid system

X chromosomesEarly embryo:Allele fororange fur

Allele forblack fur

Cell division andX chromosomeinactivation

Two cellpopulationsin adult cat:

Active X

Active XInactive X

Black fur Orange fur

X Inactivation in Female Mammals

• In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development

• The inactive X condenses into a Barr body

• If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

Alterations of Chromosome Structure

• Breakage of a chromosome can lead to four types of changes in chromosome structure:– Deletion removes a chromosomal segment– Duplication repeats a segment– Inversion reverses a segment within a chromosome– Translocation moves a segment from one

chromosome to another

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

DeletionA B C D E F G H A B C E F G H(a)

(b)

(c)

(d)

Duplication

Inversion

Reciprocaltranslocation

A B C D E F G H

A B C D E F G H

A B C D E F G H

A B C B C D E F G H

A D C B E F G H

M N O C D E F G H

M N O P Q R A B P Q R

DNA Technology

TRANSFORMATION

Protein Synthesis

• The making of Proteins!• DNA carries the

“blueprints” for the making of every molecule and cellular structure.

• Transcription• Translation